1. Field of the Invention
This invention relates to a method for reforming a sulfur-containing carbonaceous fuel to produce a hydrogen-rich gas suitable for use in fuel cell power generating systems or other systems which generally are not sulfur-tolerant and a catalyst composition for use in the method. The catalyst is a multi-part reforming catalyst comprising a dehydrogenation portion, an oxidation portion and a hydrodesulfurization portion.
2. Description of Prior Art
A fuel cell is an electrochemical device comprising an anode electrode, a cathode electrode and an electrolyte disposed between the anode electrode and the cathode electrode. Individual fuel cells or fuel cell units typically are stacked with bipolar separator plates separating the anode electrode of one fuel cell unit from the cathode electrode of an adjacent fuel cell unit to produce fuel cell stacks. There are four basic types of fuel cells, molten carbonate, phosphoric acid, solid oxide and polymer electrolyte membrane. Fuel cells typically consume a gaseous fuel and generate electricity.
Substantial advancements have been made during the past several years in fuel cells for transportation, stationary and portable power generation applications.
These advancements have been spurred by the recognition that these electrochemical devices have the potential for high efficiency and lower emissions than conventional power producing equipment. Increased interest in the commercialization of polymer electrolyte membrane fuel cells, in particular, has resulted from recent advances in fuel cell technology, such as the 100-fold reduction in the platinum content of the electrodes and more economical bipolar separator plates.
Ideally, polymer electrolyte membrane fuel cells operate with hydrogen. In the absence of a viable hydrogen storage option or a near-term hydrogen-refueling infrastructure, it is necessary to convert available fuels, typically CxHy and CxHyOz, collectively referred to herein as carbonaceous fuels, with a fuel processor into a hydrogen-rich gas suitable for use in fuel cells. The choice of fuel for fuel cell systems will be determined by the nature of the application and the fuel available at the point of use. In transportation applications, it may be gasoline, diesel, methanol or ethanol. In stationary systems, it is likely to be natural gas or liquified petroleum gas. In certain niche markets, the fuel could be ethanol, butane or even biomass-derived materials. In all cases, reforming of the fuel is necessary to produce a hydrogen-rich fuel.
There are basically three types of fuel processors—steam reformers, partial oxidation reformers and autothermal reformers. Most currently available fuel processors employing the steam reforming reaction are large, heavy and expensive. For fuel cell applications such as in homes, mobile homes and light-duty vehicles, the fuel processor must be compact, lightweight and inexpensive to build/manufacture and it should operate efficiently, be capable of rapid start and load following, and enable extended maintenance-free operation.
Partial oxidation and autothermal reforming best meet these requirements. However, it is preferred that the reforming process be carried out catalytically to reduce the operating temperature, which translates into lower cost and higher efficiency, and to reduce reactor volume. U.S. Pat. No. 6,110,861 to Krumpelt et al. teaches a two-part catalyst comprising a dehydrogenation portion and an oxide-ion conducting portion for partially oxidizing carbonaceous fuels such as gasoline to produce a high percentage yield of hydrogen suitable for supplying a fuel cell. The dehydrogenation portion of the catalyst is a Group VIII metal and the oxide-ion conducting portion is selected from a ceramic oxide crystallizing in the fluorite or perovskite structure. However, reforming catalysts, which are often Ni-based, are poisoned by sulfur impurities in the carbonaceous fuels, thereby requiring the addition of a hydrodesulfurization step or a sulfur adsorption bed to the fuel processor upstream of the reforming step. This is due to the adsorption of sulfur on the active metal catalyst sites. Sulfur also tends to increase coking rates, which leads to further degradation of the reforming catalysts and unacceptable catalyst performance.
Other methods for addressing this problem are known, such as U.S. Pat. No. 5,336,394 to Iino et al. which teaches a process for hydrodesulfurizing a sulfur-containing hydrocarbon in which the sulfur-containing hydrocarbon is contacted in the presence of hydrogen with a catalyst composition comprising a Group VIA metal, a Group VIII metal and an alumina under hydrodesulfurizing conditions and U.S. Pat. No. 5,270,272 to Galperin et al. which teaches a sulfur-sensitive conversion catalyst suitable for use in a reforming process in which the feedstock contains small amounts of sulfur and a method for regeneration of the catalyst. The catalyst comprises a non-acidic large-pore molecular sieve, for example, L-zeolite, an alkali-metal component and a platinum-group metal component. In addition, it may include refractory inorganic oxides such as alumina, silica, titania, magnesia, zirconia, chromia, thoria, boria or mixtures thereof, synthetically or naturally occurring clays and silicates, crystalline zeolitic aluminosilicates, spinels such as MgAl2O4, FeAl2O4, ZnAl2O4, CaAl2O4, and combinations thereof. The catalyst may also contain other metal components known to modify the effect of the preferred platinum component, such as Group IVA (14) metals, non-noble Group VIII (8-10) metals, rhenium, indium, gallium, zinc, uranium, dysprosium, thallium and mixtures thereof. However, such known methods frequently require an additional step such as regeneration of the catalyst.